V. Caínzos, M. Pérez-Hernández, D. Santana-Toscano, C. Arumí-Planas, A. Hernández‐Guerra
Abstract. The circulation in the Atlantic Ocean is marked by the complex system of pathways of the Atlantic Meridional Overturning Circulation (AMOC). These currents change meridionally due to the interaction with nearby water masses. Hydrographic data provide the opportunity to characterize these currents for the whole water column with high-resolution data over the last 30 years. Moreover, inverse methods enable the quantification of absolute zonal transports across these sections, determining the strength of each current at a certain latitude in terms of mass, heat, and freshwater, as well as their transport-weighted temperature and salinity. Generally, no changes can be found among decades for each of the currents in terms of transport or their properties. In the South Atlantic, the circulation describes the subtropical gyre affected by several recirculations. There are nearly 61 Sv entering from the Southern and Indian oceans at 45∘ S. The South Atlantic subtropical gyre exports 17.0 ± 1.2 Sv and around 1 PW northward via the North Brazil Current, as well as −55 Sv southward at 45∘ S into the Antarctic Circumpolar Current. In the North Atlantic, most of the transport is advected northward via the western boundary currents, which reduce their strength as they take part in convection processes in the subpolar North Atlantic, also reflected in the northward progress of mass and heat transport. Deep layers carry waters southward along the western boundary, maintaining similar values of mass and heat transport until the separation into an eastern branch crossing the mid-Atlantic Ridge in the South Atlantic. Abyssal waters originating in the Southern Ocean are distributed along the South Atlantic mainly through its western subbasin, flowing northward up to 24.5∘ N, subjected to an increasing trend in their temperature with time.
{"title":"Consistent picture of the horizontal circulation of the Atlantic Ocean over 3 decades","authors":"V. Caínzos, M. Pérez-Hernández, D. Santana-Toscano, C. Arumí-Planas, A. Hernández‐Guerra","doi":"10.5194/os-19-1009-2023","DOIUrl":"https://doi.org/10.5194/os-19-1009-2023","url":null,"abstract":"Abstract. The circulation in the Atlantic Ocean is marked by the complex\u0000system of pathways of the Atlantic Meridional Overturning Circulation\u0000(AMOC). These currents change meridionally due to the interaction with\u0000nearby water masses. Hydrographic data provide the opportunity to\u0000characterize these currents for the whole water column with high-resolution\u0000data over the last 30 years. Moreover, inverse methods enable the\u0000quantification of absolute zonal transports across these sections,\u0000determining the strength of each current at a certain latitude in terms of\u0000mass, heat, and freshwater, as well as their transport-weighted temperature\u0000and salinity. Generally, no changes can be found among decades for each of\u0000the currents in terms of transport or their properties. In the South\u0000Atlantic, the circulation describes the subtropical gyre affected by several\u0000recirculations. There are nearly 61 Sv entering from the Southern and Indian\u0000oceans at 45∘ S. The South Atlantic subtropical gyre exports\u000017.0 ± 1.2 Sv and around 1 PW northward via the North Brazil Current,\u0000as well as −55 Sv southward at 45∘ S into the Antarctic Circumpolar\u0000Current. In the North Atlantic, most of the transport is advected northward\u0000via the western boundary currents, which reduce their strength as they take\u0000part in convection processes in the subpolar North Atlantic, also reflected\u0000in the northward progress of mass and heat transport. Deep layers carry\u0000waters southward along the western boundary, maintaining similar values of\u0000mass and heat transport until the separation into an eastern branch crossing\u0000the mid-Atlantic Ridge in the South Atlantic. Abyssal waters originating in\u0000the Southern Ocean are distributed along the South Atlantic mainly through its\u0000western subbasin, flowing northward up to 24.5∘ N, subjected to an\u0000increasing trend in their temperature with time.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"108 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76874335","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Basterretxea, Joan S. Font-Muñoz, I. Hernández‐Carrasco, S. Sañudo-Wilhelmy
Abstract. We examine 20 years of monthly global ocean color data and modeling outputs of nutrients using self-organizing map (SOM) analysis to identify characteristic spatial and temporal patterns of high-nutrient low-chlorophyll (HNLC) regions and their association with different climate modes. The global nitrate-to-chlorophyll ratio threshold of NO3 : Chl > 17 (mmol NO3 mg Chl−1) is estimated to be a good indicator of the distribution limit of this unproductive biome that, on average, covers 92 × 106 km2 (∼ 25 % of the ocean). The trends in satellite-derived surface chlorophyll (0.6 ± 0.4 % yr−1 to 2 ± 0.4 % yr−1) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades, but much of this variation, particularly in the Southern Ocean, is produced by a climate-driven transition in 2009–2010. Indeed, since 2010, the extent of the HNLC zones has decreased at the poles (up to 8 %) and slightly increased at the Equator (< 0.5 %). Our study finds that chlorophyll variations in HNLC regions respond to major climate variability signals such as the El Niño–Southern Oscillation (ENSO) and Meridional Overturning Circulation (MOC) at both short (2–4 years) and long (decadal) timescales. These results suggest global coupling in the functioning of distant biogeochemical regions.
{"title":"Global variability of high-nutrient low-chlorophyll regions using neural networks and wavelet coherence analysis","authors":"G. Basterretxea, Joan S. Font-Muñoz, I. Hernández‐Carrasco, S. Sañudo-Wilhelmy","doi":"10.5194/os-19-973-2023","DOIUrl":"https://doi.org/10.5194/os-19-973-2023","url":null,"abstract":"Abstract. We examine 20 years of monthly global ocean color data and\u0000modeling outputs of nutrients using self-organizing map (SOM) analysis to\u0000identify characteristic spatial and temporal patterns of high-nutrient low-chlorophyll (HNLC) regions and their association with different climate\u0000modes. The global nitrate-to-chlorophyll ratio threshold of\u0000NO3 : Chl > 17 (mmol NO3 mg Chl−1) is estimated to be a good indicator\u0000of the distribution limit of this unproductive biome that, on average,\u0000covers 92 × 106 km2 (∼ 25 % of the ocean). The\u0000trends in satellite-derived surface chlorophyll (0.6 ± 0.4 % yr−1 to 2 ± 0.4 % yr−1) suggest that HNLC regions in polar and subpolar areas\u0000have experienced an increase in phytoplankton biomass over the last decades,\u0000but much of this variation, particularly in the Southern Ocean, is produced\u0000by a climate-driven transition in 2009–2010. Indeed, since 2010, the extent\u0000of the HNLC zones has decreased at the poles (up to 8 %) and slightly\u0000increased at the Equator (< 0.5 %). Our study finds that\u0000chlorophyll variations in HNLC regions respond to major climate variability\u0000signals such as the El Niño–Southern Oscillation (ENSO) and Meridional\u0000Overturning Circulation (MOC) at both short (2–4 years) and long (decadal)\u0000timescales. These results suggest global coupling in the functioning of\u0000distant biogeochemical regions.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"103 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80795521","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ir. C. P. Keizer, D. Le Bars, Cees de Valk, A. Jüling, R. V. D. van de Wal, S. Drijfhout
Abstract. The global acceleration of sea-level rise (SLR) during the 20th century is now established. On the local scale, this is harder to establish as several drivers of SLR play a role, which can mask the acceleration. Here, we study the rate of SLR along the coast of the Netherlands from the average of six tide gauge records covering the period 1890–2021. To isolate the effects of the wind field variations and the nodal tide from the local sea-level trend, we use four generalised additive models (GAMs) which include different predictive variables. From the sea-level trend estimates, we obtain the continuous evolution of the rate of SLR and its uncertainty over the observational period. The standard error in the estimation of the rate of SLR is reduced when we account for nodal-tide effects and is reduced further when we also account for the wind effects, meaning these provide better estimates of the rate of SLR. A part of the long-term SLR is due to wind forcing related to a strengthening and northward shift of the jet stream, but this SLR contribution decelerated over the observational period. Additionally, we detect wind-forced sea-level variability on multidecadal timescales with an amplitude of around 1 cm. Using a coherence analysis, we identify both the North Atlantic Oscillation and the Atlantic Multidecadal Variability as its drivers. Crucially, accounting for the nodal-tide and wind effects changes the estimated rate of SLR, unmasking an SLR acceleration that started in the 1960s. Our best-fitting GAM, which accounts for nodal and wind effects, yields a rate of SLR of about 1.72.21.3 mm yr−1 in 1900–1919 and 1.51.91.2 mm yr−1 in 1940–1959 compared to 2.93.52.4 mm yr−1 in 2000–2019 (where the lower and upper bounds denote the 5th and 95th percentiles). If we discount the nodal tide, wind and fluctuation effects and assume a constant rate of SLR, then the probability (p value) of finding a rate difference between 1940–1959 and 2000–2019 of at least our estimate is smaller than 1 %. Consistent with global observations and the expectations based on the physics of global warming, our results show unequivocally that SLR along the Dutch coast has accelerated since the 1960s.
摘要20世纪全球海平面上升加速(SLR)已经确定。在局部范围内，这很难确定，因为单反的几个驱动因素起了作用，这可能掩盖了加速。在这里，我们根据1890-2021年期间六个潮汐计记录的平均值研究了荷兰沿海的SLR率。为了从局地海平面趋势中分离出风场变化和节点潮的影响，我们使用了四种包含不同预测变量的广义加性模式(GAMs)。从海平面趋势估计中，我们得到了SLR率在观测期内的连续演变及其不确定性。当我们考虑到节点潮汐效应时，估计单反率的标准误差会降低，当我们考虑到风的影响时，估计的标准误差会进一步降低，这意味着这些提供了更好的单反率估计。部分长期SLR是由于与急流加强和向北移动有关的风强迫，但这种SLR的贡献在观测期间有所减弱。此外，我们在多年代际时间尺度上探测到风引起的海平面变化，幅度约为1厘米。通过相干分析，我们确定北大西洋涛动和大西洋多年代际变率都是其驱动因素。至关重要的是，考虑到节点潮和风的影响，会改变单反的估计速率，从而揭示了始于20世纪60年代的单反加速。我们的最佳拟合GAM(考虑了节点和风的影响)得出的SLR率在1900-1919年为1.72.21.3 mm yr - 1，在1940-1959年为1.51.91.2 mm yr - 1，而在2000-2019年为2.93.52.4 mm yr - 1(其中下界和上界分别表示第5和第95百分位数)。如果我们不考虑节点潮、风和波动的影响，并假设SLR的速率恒定，那么至少在我们的估计中，在1940-1959年和2000-2019年之间发现速率差异的概率(p值)小于1%。与全球观测和基于全球变暖物理学的预期相一致，我们的结果明确表明，自20世纪60年代以来，荷兰海岸的单反加速了。
{"title":"The acceleration of sea-level rise along the coast of the Netherlands started in the 1960s","authors":"Ir. C. P. Keizer, D. Le Bars, Cees de Valk, A. Jüling, R. V. D. van de Wal, S. Drijfhout","doi":"10.5194/os-19-991-2023","DOIUrl":"https://doi.org/10.5194/os-19-991-2023","url":null,"abstract":"Abstract. The global acceleration of sea-level rise (SLR) during the 20th century is now established.\u0000On the local scale, this is harder to establish as several drivers of SLR play a role, which can mask the acceleration.\u0000Here, we study the rate of SLR along the coast of the Netherlands from the average of six tide gauge records covering the period 1890–2021.\u0000To isolate the effects of the wind field variations and the nodal tide from the local sea-level trend, we use four generalised additive models (GAMs) which include different predictive variables.\u0000From the sea-level trend estimates, we obtain the continuous evolution of the rate of SLR and its uncertainty over the observational period.\u0000The standard error in the estimation of the rate of SLR is reduced when we account for nodal-tide effects and is reduced further when we also account for the wind effects, meaning these provide better estimates of the rate of SLR.\u0000A part of the long-term SLR is due to wind forcing related to a strengthening and northward shift of the jet stream, but this SLR contribution decelerated over the observational period.\u0000Additionally, we detect wind-forced sea-level variability on multidecadal timescales with an amplitude of around 1 cm.\u0000Using a coherence analysis, we identify both the North Atlantic Oscillation and the Atlantic Multidecadal Variability as its drivers.\u0000Crucially, accounting for the nodal-tide and wind effects changes the estimated rate of SLR, unmasking an SLR acceleration that started in the 1960s.\u0000Our best-fitting GAM, which accounts for nodal and wind effects, yields a rate of SLR of about 1.72.21.3 mm yr−1 in 1900–1919 and 1.51.91.2 mm yr−1 in 1940–1959 compared to 2.93.52.4 mm yr−1 in 2000–2019 (where the lower and upper bounds denote the 5th and 95th percentiles).\u0000If we discount the nodal tide, wind and fluctuation effects and assume a constant rate of SLR, then the probability (p value) of finding a rate difference between 1940–1959 and 2000–2019 of at least our estimate is smaller than 1 %.\u0000Consistent with global observations and the expectations based on the physics of global warming, our results show unequivocally that SLR along the Dutch coast has accelerated since the 1960s.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"15 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85204339","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. We analyzed physical oceanic parameters gathered by a mooring array at mesoscale spatial sampling deployed in the Argentine Basin within the Ocean Observatory Initiative, a National Science Foundation major research facility. The array was maintained at 42∘ S and 42∘ W, a historically sparsely sampled region with small ocean variability, over 34 months from March 2015 to January 2018. The data documented four anticyclonic extreme-structure events in 2016. The four anticyclonic structures had different characteristics (size, vertical extension, origin, lifetime and Rossby number). They all featured near-inertial waves (NIWs) trapped at depth and low Richardson values well below the mixed layer. Low Richardson values suggest favorable conditions for mixing. The anticyclonic features likely act as mixing structures at the pycnocline, bringing heat and salt from the South Atlantic Central Water to the Antarctic Intermediate Waters. The intense structures were unique in the 29-year-long satellite altimetry record at the mooring site. The Argentine Basin is populated with many anticyclones, and mixing associated with trapped NIWs probably plays an important role in setting up the upper-water-mass characteristics in the basin.
{"title":"Intense anticyclones at the global Argentine Basin array of the Ocean Observatory Initiative","authors":"C. Artana, C. Provost","doi":"10.5194/os-19-953-2023","DOIUrl":"https://doi.org/10.5194/os-19-953-2023","url":null,"abstract":"Abstract. We analyzed physical oceanic parameters gathered by a mooring array at mesoscale spatial sampling deployed in the Argentine Basin within the Ocean Observatory Initiative, a National Science Foundation major research facility. The array was maintained at 42∘ S and 42∘ W, a historically sparsely sampled region with small ocean variability, over 34 months from March 2015 to January 2018. The data documented four anticyclonic extreme-structure events in 2016. The four anticyclonic structures had different characteristics (size, vertical extension, origin, lifetime and Rossby number). They all featured near-inertial waves (NIWs) trapped at depth and low Richardson values well below the mixed layer. Low Richardson values suggest favorable conditions for mixing. The anticyclonic features likely act as mixing structures at the pycnocline, bringing heat and salt from the South Atlantic Central Water to the Antarctic Intermediate Waters. The intense structures were unique in the 29-year-long satellite altimetry record at the mooring site. The Argentine Basin is populated with many anticyclones, and mixing associated with trapped NIWs probably plays an important role in setting up the upper-water-mass characteristics in the basin.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"101 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85970517","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. Equatorial deep jets (EDJs) are vertically alternating, stacked zonal currents that flow along the Equator in all three ocean basins at intermediate depth. Their structure can be described quite well by the sum of high-baroclinic-mode equatorial Kelvin and Rossby waves. However, the EDJ meridional width is larger by a factor of 1.5 than inviscid theory predicts for such waves. Here, we use a set of idealised model configurations representing the Atlantic Ocean to investigate the contributions of different processes to the enhanced EDJ width. Corroborated by the analysis of shipboard velocity sections, we show that widening of the EDJs by momentum loss due to irreversible mixing or other processes contributes more to their enhanced time mean width than averaging over meandering of the jets. Most of the widening due to meandering can be attributed to the strength of intraseasonal variability in the jets' depth range, suggesting that the jets are meridionally advected by intraseasonal waves. A slightly weaker connection to intraseasonal variability is found for the EDJ widening by momentum loss. These results enhance our understanding of the dynamics of the EDJs and, more generally, of equatorial waves in the deep ocean.
{"title":"Factors influencing the meridional width of the equatorial deep jets","authors":"S. Bastin, M. Claus, R. Greatbatch, P Brandt","doi":"10.5194/os-19-923-2023","DOIUrl":"https://doi.org/10.5194/os-19-923-2023","url":null,"abstract":"Abstract. Equatorial deep jets (EDJs) are vertically alternating, stacked zonal currents that flow along the Equator in all three ocean basins at intermediate depth. Their structure can be described quite well by the sum of high-baroclinic-mode equatorial Kelvin and Rossby waves. However, the EDJ meridional width is larger by a factor of 1.5 than inviscid theory predicts for such waves. Here, we use a set of idealised model configurations representing the Atlantic Ocean to investigate the contributions of different processes to the enhanced EDJ width. Corroborated by the analysis of shipboard velocity sections, we show that widening of the EDJs by momentum loss due to irreversible mixing or other processes contributes more to their enhanced time mean width than averaging over meandering of the jets. Most of the widening due to meandering can be attributed to the strength of intraseasonal variability in the jets' depth range, suggesting that the jets are meridionally advected by intraseasonal waves. A slightly weaker connection to intraseasonal variability is found for the EDJ widening by momentum loss. These results enhance our understanding of the dynamics of the EDJs and, more generally, of equatorial waves in the deep ocean.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"16 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73609037","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
I. Parras-Berrocal, R. Vázquez, W. Cabos, D. Sein, O. Álvarez, M. Bruno, A. Izquierdo
Abstract. Dense water formation in the eastern Mediterranean (EMed) is essential in sustaining the Mediterranean overturning circulation. Changes in the sources of dense water in the EMed point to changes in the circulation and water properties of the Mediterranean Sea. Here we examine with a regional climate system model the changes in the dense water formation in the EMed through the 21st century under the RCP8.5 emission scenario. Our results show a shift in the dominant source of Eastern Mediterranean Deep Water (EMDW) from the Adriatic Sea to the Aegean Sea in the first half of the 21st century. The projected dense water formation is reduced by 75 % for the Adriatic Sea, 84 % for the Aegean Sea, and 83 % for the Levantine Sea by the end of the century. The reduction in the intensity of deep water formation is related to hydrographic changes in surface and intermediate water that strengthen the vertical stratification, hampering vertical mixing and thus convection. Those changes have an impact on the water that flows through the Strait of Sicily to the western Mediterranean and therefore on the whole Mediterranean system.
{"title":"Dense water formation in the eastern Mediterranean under a global warming scenario","authors":"I. Parras-Berrocal, R. Vázquez, W. Cabos, D. Sein, O. Álvarez, M. Bruno, A. Izquierdo","doi":"10.5194/os-19-941-2023","DOIUrl":"https://doi.org/10.5194/os-19-941-2023","url":null,"abstract":"Abstract. Dense water formation in the eastern Mediterranean (EMed)\u0000is essential in sustaining the Mediterranean overturning circulation.\u0000Changes in the sources of dense water in the EMed point to changes in the\u0000circulation and water properties of the Mediterranean Sea. Here we\u0000examine with a regional climate system model the changes in the dense water\u0000formation in the EMed through the 21st century under the RCP8.5\u0000emission scenario. Our results show a shift in the dominant source of\u0000Eastern Mediterranean Deep Water (EMDW) from the Adriatic Sea to the Aegean\u0000Sea in the first half of the 21st century. The projected dense water\u0000formation is reduced by 75 % for the Adriatic Sea, 84 % for the Aegean Sea,\u0000and 83 % for the Levantine Sea by the end of the century. The reduction in\u0000the intensity of deep water formation is related to hydrographic changes in\u0000surface and intermediate water that strengthen the vertical stratification,\u0000hampering vertical mixing and thus convection. Those changes have an\u0000impact on the water that flows through the Strait of Sicily to the western\u0000Mediterranean and therefore on the whole Mediterranean system.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"29 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85234614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. Over the last 3 decades, satellite altimetry has observed sea surface height variations, providing a regular monitoring of the surface ocean circulation. Altimetry measurements have an intrinsic signal-to-noise ratio that limits the spatial scales of the currents that can be captured. However, the recent progress made on both altimetry sensors and data processing allows us to observe smaller geophysical signals, offering new perspectives in coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter missions with three different technologies to capture the Northern Current (northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic aperture radar altimeter, launched in 2016). Therefore, we use a high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the model is very close to the observations of high-frequency radars and gliders in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on average, with a mean core velocity of 0.39 m s−1. Its signature in terms of sea level consists of a drop whose mean value at 6.14∘ E is 6.9 cm, extending over 20 km. These variations show a clear seasonal pattern, but high-frequency signals are also present most of the time. In comparison, in 1 Hz altimetry data, the mean sea level drop associated with the Northern Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of corresponding sea level variability, Jason-2 and SARAL altimetry estimates are larger than the model reference (+1.3 and +1 cm respectively), whereas Sentinel-3A shows closer values (−0.4 cm). When we derive geostrophic surface currents from the satellite sea level variations without any data filtering, in comparison to the model, the standard deviations of the velocity values are also very different from one mission to the other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea level data with different cutoff wavelengths, the best agreement between the model and the altimetry distributions of velocity values are obtained with a 60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to illustrate how the advances in the performances of altimeters and in the data processing improve the observation of coastal currents but also to quantify the corresponding gain.
摘要在过去的30年里，卫星测高观测到了海面高度的变化，提供了对海面海洋环流的定期监测。测高测量具有固有的信噪比，这限制了可以捕获的电流的空间尺度。然而，最近在测高传感器和数据处理方面取得的进展使我们能够观察到较小的地球物理信号，为这些结构重要的沿海地区提供了新的视角。在这项方学研究中，我们评估了采用三种不同技术的三个测高仪捕获北流(地中海西北部)及其变率的能力，即Jason-2(2008年发射的ku波段低分辨率模式测高仪)、SARAL/AltiKa(2013年发射的ka波段低分辨率模式测高仪)和Sentinel-3A(2016年发射的合成孔径雷达测高仪)。因此，我们使用高分辨率区域模型作为参考。我们将重点放在法国普罗旺斯海岸，在那里我们首次表明，该模型非常接近高频雷达和滑翔机的观测结果，就表面电流估计而言。在模型中，北流在距离海岸15-20公里处被观测到，平均核心速度为0.39 m s - 1。它在海平面上的特征是在6.14°E处的平均落差为6.9厘米，延伸超过20公里。这些变化显示出明显的季节性模式，但大多数时候也存在频率信号。相比之下，在1hz高程数据中，Jason-2与北流相关的平均海平面下降高估了3.0 cm，但saral /AltiKa和Sentinel-3A的高估幅度明显较小(分别为0.3 cm和1.4 cm)。就相应的海平面变率而言，Jason-2和SARAL的测高估计值比模式参考值大(分别为+1.3和+1 cm)，而Sentinel-3A的值更接近(- 0.4 cm)。当我们在没有任何数据过滤的情况下从卫星海平面变化中推导出营养化表面流时，与模型相比，速度值的标准差在不同任务之间也有很大差异(Jason-2的标准差大3.7倍，而saral和Sentinel-3A的标准差分别大2.4倍和2.9倍)。在低通滤波不同截止波长的高程海平面数据时，Jason-2、SARAL和sentinel - 3a数据分别在60、30和40-50 km截止波长处的速度值与模型的高程分布最吻合。这项研究表明，使用高分辨率模型作为高度计数据的参考，不仅可以说明高度计性能和数据处理方面的进步如何改善沿海洋流的观测，而且可以量化相应的增益。
{"title":"Assessing the capability of three different altimetry satellite missions to observe the Northern Current by using a high-resolution model","authors":"A. Carret, F. Birol, C. Estournel, B. Zakardjian","doi":"10.5194/os-19-903-2023","DOIUrl":"https://doi.org/10.5194/os-19-903-2023","url":null,"abstract":"Abstract. Over the last 3 decades, satellite altimetry has observed sea surface\u0000height variations, providing a regular monitoring of the surface ocean\u0000circulation. Altimetry measurements have an intrinsic signal-to-noise ratio\u0000that limits the spatial scales of the currents that can be captured. However,\u0000the recent progress made on both altimetry sensors and data processing allows\u0000us to observe smaller geophysical signals, offering new perspectives in\u0000coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter\u0000missions with three different technologies to capture the Northern Current\u0000(northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic\u0000aperture radar altimeter, launched in 2016). Therefore, we use a\u0000high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the\u0000model is very close to the observations of high-frequency radars and gliders\u0000in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on\u0000average, with a mean core velocity of 0.39 m s−1. Its signature in terms of sea\u0000level consists of a drop whose mean value at 6.14∘ E is 6.9 cm,\u0000extending over 20 km. These variations show a clear seasonal pattern, but\u0000high-frequency signals are also present most of the time. In comparison, in\u00001 Hz altimetry data, the mean sea level drop associated with the Northern\u0000Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with\u0000SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of\u0000corresponding sea level variability, Jason-2 and SARAL altimetry estimates\u0000are larger than the model reference (+1.3 and +1 cm respectively),\u0000whereas Sentinel-3A shows closer values (−0.4 cm). When we derive\u0000geostrophic surface currents from the satellite sea level variations\u0000without any data filtering, in comparison to the model, the standard\u0000deviations of the velocity values are also very different from one mission to the\u0000other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for\u0000SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea\u0000level data with different cutoff wavelengths, the best agreement between the\u0000model and the altimetry distributions of velocity values are obtained with a\u000060, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and\u0000Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to\u0000illustrate how the advances in the performances of altimeters and in the\u0000data processing improve the observation of coastal currents but also to\u0000quantify the corresponding gain.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"1 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79668288","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Emmanuel Romero, L. Tenorio-Fernandez, E. Portela, J. Montes-Aréchiga, L. Sánchez‐Velasco
Abstract. According to the typical thermal structure of the ocean, the water column can be divided into three layers: the mixed layer, the thermocline and the deep layer. In this study, we provide a new methodology, based on a function adjustment to the temperature profile, to locate the minimum and maximum depths of the strongest thermocline. We first validated our methodology by comparing the mixed layer depth obtained with the method proposed here with three other methods from previous studies. Since we found a very good agreement between the four methods we used the function adjustment to compute the monthly climatologies of the maximum thermocline depth and the thermocline thickness and strength in the global ocean. We also provide an assessment of the regions of the ocean where our adjustment is valid, i.e., where the thermal structure of the ocean follows the three-layer structure. However, there are ocean regions where the water column cannot be separated into three layers due to the dynamic processes that alter it. This assessment highlights the limitations of the existing methods to accurately determine the mixed layer depth and the thermocline depth in oceanic regions that are particularly turbulent such as the Southern Ocean and the northern North Atlantic, among others. The method proposed here has shown to be robust and easy to apply.
{"title":"Improving the thermocline calculation over the global ocean","authors":"Emmanuel Romero, L. Tenorio-Fernandez, E. Portela, J. Montes-Aréchiga, L. Sánchez‐Velasco","doi":"10.5194/os-19-887-2023","DOIUrl":"https://doi.org/10.5194/os-19-887-2023","url":null,"abstract":"Abstract. According to the typical thermal structure of the ocean, the water column can be divided into three layers: the mixed layer, the thermocline and the deep layer. In this study, we provide a new methodology, based on a function adjustment to the temperature profile, to locate the minimum and maximum depths of the strongest thermocline. We first validated our methodology by comparing the mixed layer depth obtained with the method proposed here with three other methods from previous studies. Since we found a very good agreement between the four methods we used the function adjustment to compute the monthly climatologies of the maximum thermocline depth and the thermocline thickness and strength in the global ocean. We also provide an assessment of the regions of the ocean where our adjustment is valid, i.e., where the thermal structure of the ocean follows the three-layer structure. However, there are ocean regions where the water column cannot be separated into three layers due to the dynamic processes that alter it. This assessment highlights the limitations of the existing methods to accurately determine the mixed layer depth and the thermocline depth in oceanic regions that are particularly turbulent such as the Southern Ocean and the northern North Atlantic, among others. The method proposed here has shown to be robust and easy to apply.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"29 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76313233","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Dani C. Jones, Maike Sonnewald, Shenjie Zhou, Ute Hausmann, Andrew J. S. Meijers, Isabella Rosso, Lars Boehme, Michael P. Meredith, Alberto C. Naveira Garabato
Abstract. The Weddell Gyre is a major feature of the Southern Ocean and an important component of the planetary climate system; it regulates air–sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterised by low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification that stabilises the water column. Observing the Weddell Gyre is challenging, as it is extremely remote and largely covered with sea ice. At present, it is one of the most poorly sampled regions of the global ocean, highlighting the need to extract as much value as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, the PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and the Weddell Gyre, (3) a gyre edge class with northern and southern branches, and (4) a gyre core class. PCM highlights, in an objective and interpretable way, both expected and underappreciated structures in the Weddell Gyre dataset. For instance, PCM identifies the inflow of Circumpolar Deep Water (CDW) across the eastern boundary, the presence of the Weddell–Scotia Confluence waters, and structured spatial variability in mixing between Winter Water and CDW. PCM offers a useful complement to existing expertise-driven approaches for characterising the physical configuration and variability of oceanographic regions, helping to identify coherent thermohaline structures and the boundaries between them.
{"title":"Unsupervised classification identifies coherent thermohaline structures in the Weddell Gyre region","authors":"Dani C. Jones, Maike Sonnewald, Shenjie Zhou, Ute Hausmann, Andrew J. S. Meijers, Isabella Rosso, Lars Boehme, Michael P. Meredith, Alberto C. Naveira Garabato","doi":"10.5194/os-19-857-2023","DOIUrl":"https://doi.org/10.5194/os-19-857-2023","url":null,"abstract":"Abstract. The Weddell Gyre is a major feature of the Southern Ocean and an important component of the planetary climate system; it regulates air–sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterised by low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification that stabilises the water column. Observing the Weddell Gyre is challenging, as it is extremely remote and largely covered with sea ice. At present, it is one of the most poorly sampled regions of the global ocean, highlighting the need to extract as much value as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, the PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and the Weddell Gyre, (3) a gyre edge class with northern and southern branches, and (4) a gyre core class. PCM highlights, in an objective and interpretable way, both expected and underappreciated structures in the Weddell Gyre dataset. For instance, PCM identifies the inflow of Circumpolar Deep Water (CDW) across the eastern boundary, the presence of the Weddell–Scotia Confluence waters, and structured spatial variability in mixing between Winter Water and CDW. PCM offers a useful complement to existing expertise-driven approaches for characterising the physical configuration and variability of oceanographic regions, helping to identify coherent thermohaline structures and the boundaries between them.","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"317 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136249534","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Sims, Mohamed M. M. Ahmed, Brian J. Butterworth, P. Duke, S. Gonski, S. Jones, K. Brown, C. Mundy, W. Williams, B. Else
Abstract. Warming of the Arctic due to climate change means the Arctic Ocean is now free from ice for longer, as sea ice melts earlier and refreezes later. Yet, it remains unclear how this extended ice-free period will impact carbon dioxide (CO2) fluxes due to scarcity of surface ocean CO2 measurements. Baseline measurements are urgently needed to understand spatial and temporal air–sea CO2 flux variability in the changing Arctic Ocean. There is also uncertainty as to whether the previous basin-wide surveys are representative of the many smaller bays and inlets that make up the Canadian Arctic Archipelago (CAA). By using a research vessel that is based in the remote Inuit community of Ikaluqtuutiak (Cambridge Bay, Nunavut), we have been able to reliably survey pCO2 shortly after ice melt and access previously unsampled bays and inlets in the nearby region. Here we present 4 years of consecutive summertime pCO2 measurements collected in the Kitikmeot Sea in the southern CAA. Overall, we found that this region is a sink for atmospheric CO2 in August (average of all calculated fluxes over the four cruises was −4.64 mmol m−2 d−1), but the magnitude of this sink varies substantially between years and locations (average calculated fluxes of +3.58, −2.96, −16.79 and −0.57 mmol m−2 d−1 during the 2016, 2017, 2018 and 2019 cruises, respectively). Surface ocean pCO2 varied by up to 156 µatm between years, highlighting the importance of repeat observations in this region, as this high interannual variability would not have been captured by sparse and infrequent measurements. We find that the surface ocean pCO2 value at the time of ice melt is extremely important in constraining the magnitude of the air–sea CO2 flux throughout the ice-free season. However, further constraining the air–sea CO2 flux in the Kitikmeot Sea will require a better understanding of how pCO2 changes outside of the summer season. Surface ocean pCO2 measurements made in small bays and inlets of the Kitikmeot Sea were ∼ 20–40 µatm lower than in the main channels. Surface ocean pCO2 measurements made close in time to ice breakup (i.e. within 2 weeks) were ∼ 50 µatm lower than measurements made > 4 weeks after breakup. As previous basin-wide surveys of the CAA have focused on the deep shipping channels and rarely measure close to the ice breakup date, we hypothesize that there may be an observational bias in previous studies, leading to an underestimate of the CO2 sink in the CAA. These high-resolution measurements constitute an important new baseline for gaining a better understanding of the role this region plays in the uptake of atmospheric CO2.
{"title":"High interannual surface pCO2 variability in the southern Canadian Arctic Archipelago's Kitikmeot Sea","authors":"R. Sims, Mohamed M. M. Ahmed, Brian J. Butterworth, P. Duke, S. Gonski, S. Jones, K. Brown, C. Mundy, W. Williams, B. Else","doi":"10.5194/os-19-837-2023","DOIUrl":"https://doi.org/10.5194/os-19-837-2023","url":null,"abstract":"Abstract. Warming of the Arctic due to climate change means the\u0000Arctic Ocean is now free from ice for longer, as sea ice melts earlier and\u0000refreezes later. Yet, it remains unclear how this extended ice-free period\u0000will impact carbon dioxide (CO2) fluxes due to scarcity of surface\u0000ocean CO2 measurements. Baseline measurements are urgently needed to\u0000understand spatial and temporal air–sea CO2 flux variability in the\u0000changing Arctic Ocean. There is also uncertainty as to whether the previous\u0000basin-wide surveys are representative of the many smaller bays and inlets\u0000that make up the Canadian Arctic Archipelago (CAA). By using a research\u0000vessel that is based in the remote Inuit community of Ikaluqtuutiak\u0000(Cambridge Bay, Nunavut), we have been able to reliably survey pCO2\u0000shortly after ice melt and access previously unsampled bays and inlets in\u0000the nearby region. Here we present 4 years of consecutive summertime\u0000pCO2 measurements collected in the Kitikmeot Sea in the southern CAA.\u0000Overall, we found that this region is a sink for atmospheric CO2 in\u0000August (average of all calculated fluxes over the four cruises was −4.64 mmol m−2 d−1), but the magnitude of this sink varies substantially\u0000between years and locations (average calculated fluxes of +3.58, −2.96,\u0000−16.79 and −0.57 mmol m−2 d−1 during the 2016, 2017, 2018 and 2019\u0000cruises, respectively). Surface ocean pCO2 varied by up to 156 µatm\u0000between years, highlighting the importance of repeat observations in this\u0000region, as this high interannual variability would not have been captured by\u0000sparse and infrequent measurements. We find that the surface ocean\u0000pCO2 value at the time of ice melt is extremely important in\u0000constraining the magnitude of the air–sea CO2 flux throughout the\u0000ice-free season. However, further constraining the air–sea CO2 flux\u0000in the Kitikmeot Sea will require a better understanding of how pCO2\u0000changes outside of the summer season. Surface ocean pCO2 measurements\u0000made in small bays and inlets of the Kitikmeot Sea were ∼ 20–40 µatm lower than in the main channels. Surface ocean pCO2\u0000measurements made close in time to ice breakup (i.e. within 2 weeks) were\u0000∼ 50 µatm lower than measurements made > 4\u0000weeks after breakup. As previous basin-wide surveys of the CAA have focused\u0000on the deep shipping channels and rarely measure close to the ice breakup\u0000date, we hypothesize that there may be an observational bias in previous\u0000studies, leading to an underestimate of the CO2 sink in the CAA. These\u0000high-resolution measurements constitute an important new baseline for\u0000gaining a better understanding of the role this region plays in the uptake\u0000of atmospheric CO2.\u0000","PeriodicalId":19535,"journal":{"name":"Ocean Science","volume":"64 2 1","pages":""},"PeriodicalIF":3.2,"publicationDate":"2023-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76701368","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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